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The "giant'' radio galaxies (GRGs), which we define as double radio sources whose overall projected linear extents exceed 2h50-1 Mpc, are interesting as extreme examples of radio source development and evolution. Members of this class, which comprise only a few percent of all powerful extragalactic radio sources, have been documented for almost 25 years (e.g., [Willis, Strom, & Wilson 1974]). They have been used to constrain the spectral aging and evolution of radio sources and as tests for the evolution of conditions in intergalactic environments on Mpc scales ([Strom & Willis 1980]; [Subrahmanyan & Saripalli 1993]; [Cotter, Rawlings, & Saunders 1996]). Their 1.4 GHz radio powers are generally in the regime 1024.5 < P1.4 < 1026 h50-2 W Hz-1 , just above the transition between Fanaroff-Riley Types I (plumed) and II (lobed) radio structures ([Fanaroff & Riley 1974]). It is unclear whether the giant sources are examples of unusually long-lived (and directionally stable) nuclear activity in radio-loud systems, or of the development of sources in unusually low-density environments.

Because of their large angular sizes, nearby giant radio galaxies can in principle be studied in great detail, but their largest-scale structures may be over-resolved and undersampled by interferometers. They have traditionally been discovered through sky surveys with compact interferometers or single dishes at relatively low frequencies, where angular resolution is modest but large fields of view and diffuse steep-spectrum structures can be imaged more easily. The source NVSS 2146+82 was noted as a candidate giant radio galaxy when it appeared in the first 4° by 4° field surveyed by the NRAO VLA Sky Survey (NVSS: [Condon et al. 1998]), a northern-hemisphere survey at 1.4 GHz using the VLA D configuration at 45'' (FWHM) resolution.

Figure 1 shows contours of the NVSS image at 45'' resolution. There are two symmetric, extended lobes (D and E) on either side of an unresolved component C, plus an unusually large number of other radio sources within 10' of C. Two of these (A and B) are also symmetrically located around C.

Figure 1: Contour plot of the NVSS 1.4 GHz total intensity data for the field. Contours are shown at -1, 1, 2,4, 8, 16, and 32 mJy per CLEAN beam area.

Comparison with the Digital Sky Survey (DSS) showed that source C coincides with an ~ 18th mag elliptical galaxy to within the uncertainties in the NVSS and DSS positions. If the elliptical galaxy is the host of an unusually large radio source (C+D+E), then the apparent magnitude suggests that the whole structure may be similar in linear scale to 3C236. The DSS also shows a nearby image that might be another galactic nucleus, and a faint extended feature suggesting a possible "tail'' or interaction.

We have undertaken several observational studies of the radio and optical objects in the field to determine their nature and to clarify the relationships between the optical and radio sources. These studies include:

  1. High resolution radio imaging at 4.9 and 8.4 GHz to locate any compact flat-spectrum radio components in the field, and thus to identify any AGN that could be responsible for some or all of the other radio emission,
  2. A search for fainter diffuse radio emission between the D and E components that might link them together or to other sources in the field and thus clarify their physical relationship,
  3. Higher-resolution radio imaging of the other radio sources in the field to explore whether they might be physically related to the diffuse components, or to each other by gravitational lensing,
  4. Optical spectroscopy of both optical "nuclei'' and other galaxies in the field,
  5. UBVRI optical photometry of the field, and
  6. X-ray imaging using ROSAT HRI observations to search for any hot X-ray emitting gas which might be associated with an overdensity of galaxies or non-thermal X-ray emission from an AGN.

Throughout this paper, we assume a Hubble constant H0 = 50h50 km s-1 Mpc-1 . At a redshift of z = 0.145, the angular diameter distance to the radio galaxy is 708.4h50-1 Mpc, the luminosity distance is 928.7h50-1 Mpc, and 1' corresponds to 206h50-1 kpc.

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Next: Radio Observations Up: Multiwavelength Observations of the Second Largest Known FRII Previous: Multiwavelength Observations of the Second Largest Known FRII

Fri Feb 4 16:02:06 EST 2000